1. Field of the Invention
The present invention relates to an X-ray image processing technique.
2. Description of the Related Art
An X-ray fluoroscopic image has a large amount of random noise because radiography is performed with a very low dose from the viewpoint of exposure. Accordingly, performing a noise suppressing process in X-ray fluoroscopic imaging is very important to increase the clearness of a diagnostic image.
To suppress noise, it is necessary to accurately estimate the noise amount on an image. If the noise suppressing process is carelessly performed, the edges of an X-ray fluoroscopic image may be blurred, or the noise cannot be suppressed at all.
As a technique of estimating an accurate noise amount on an image, an object in each motion image frame is analyzed and the noise amount of each object is estimated in Japanese Patent No. 3762725. Although Japanese Patent No. 3762725 takes a general video camera application as an example, the same effect may be obtained in an X-ray image. The larger the thickness of an object, the more difficult it becomes to transmit X-rays. Since this reduces the X-ray dose reaching an X-ray receiving sensor, an image having a large amount of noise relative to the signal value is obtained. Accordingly, the method of estimating the noise amount of each object is effective.
The random noise of X-rays is classified into random quantum noise Nq caused by X-rays and random system noise Ns as electrical noise in an X-ray receiving sensor. These two types of random noise are added as random noise σ(X) onto an X-ray image. σ(X) is represented by
                              σ          ⁡                      (            X            )                          =                                            N              q              2                        +                          N              s              2                                                          (        1        )                                          N          q                =                              K            q                    ·                                    (              X              )                                      1              2                                                          (        2        )            
FIG. 3 shows the above equations as the relationship between the X-ray dose (X-ray irradiation intensity) and X-ray random noise amount. This relationship shows that when the dose of an X-ray image is low, the influence of the random system noise is larger than that of the random quantum noise. The technique disclosed in Japanese Patent No. 3762725 can estimate only the random quantum noise caused by X-rays because the method is object analysis. Therefore, Japanese Patent Laid-Open No. 2005-006792 has proposed a detection method of detecting the random system noise by using an AC magnetic field detector. In addition, Japanese Patent Laid-Open No. 2004-023764 has proposed a method of storing a reference noise pattern in accordance with radiography conditions, and loading the noise pattern in accordance with the radiography conditions.
To increase the clearness of an X-ray image, however, it is necessary to accurately obtain the noise amount of the image, and process the X-ray image in accordance with the noise amount.